1. Field of the Invention
The present invention relates to cable bolts for use in the mining and construction industries to support the walls and roofs of underground excavations and openings and particularly, although not exclusively, to cable bolts for use in coal mining where the softer excavation material generally requires differing considerations to those in metalliferous mining.
2. Description of the Related Art
In coal mines the cable bolt is normally fixed at the inner end of a hole drilled in the mine roof by means of a two part quick curing resin. The epoxy resin is inserted into the hole in separate packaged parts and pushed to the end of the hole by insertion of the cable bolt which also causes destruction of the packaging and mixing of the parts. Rotation of the bolt during full insertion ensures proper mixing of the resin which quickly cures to secure the top of the cable bolt in the hole. In some circumstances cement grouting is then pumped into the hole to secure the bolt along its entire length, to the sides of the hole.
One known form of cable bolt for coal mines is marketed under the name FLEXIBOLT(copyright), a registered trade mark of JJP Geotechnical Engineering Pty. Ltd., and comprises a number of outer wires or strands wound around a central core of smaller diameter wires or strands in a helical fashion to form a bar which constitutes the main part of the cable bolt. The lower end of the bolt, that is, the end external of the hole in use, has a thread rolled into the surface of the bolt which, because of the irregular outer surface of the bolt, is a discontinuous thread only occurring in the outermost surface of each of the outer wires or strands. A nut having a thin metal plate pressed into one end is adapted to be wound on the thread until initially the plate bears on the end of the cable bolt and prevents further progress of the nut. Further rotation of the nut by a standard drive mechanism causes rotation of the entire cable bolt to thereby mix the two part resin until the resin cures, at which time the top end of the bolt is firmly anchored and further rotation of the nut causes the plate to be forced out of the nut and the nut to progress along the threaded end of the cable bolt. A bearing plate pre-arranged on the cable bolt and a conventional barrel and wedge between the bearing plate and nut are therefore forced upwardly by the further rotation of the nut against the roof of the mine until sufficient tension is applied to the cable bolt by the rotation of the nut. While the aforementioned FLEXIBOLT(copyright) cable bolt is used quite extensively there are some features thereof which render it less than ideal. Most significantly the combination of the nut, barrel and wedge, does not normally give an anchorage force equivalent to the strength of the cable bolt, as it would require a disproportionately long barrel and wedge which, apart from cost considerations would extend downwardly into the mine an unacceptable distance. Furthermore, it is not possible with the FLEXIBOLT(copyright) device to provide additional bonding such as grouting over the remaining length of the bolt as there is no means for getting the grouting into the hole after the bearing plate and barrel and wedge are in position. Therefore, the FLEXIBOLT(copyright) device must rely only on the bonding achieved by the resin which extends about two meters down from the top of the bolt and consequently full bonding is difficult to achieve. The nut and rolled thread combination is not capable of developing more than about 30% of the ultimate tensile strength (UTS) of the cable due to the non-continuous thread and therefore the cable bolt cannot be pre-tensioned beyond about 30% of the UTS by rotation of the nut alone.
It is an object of this invention to provide a cable bolt for use in coal mines which overcomes one or more of the shortcomings of the xe2x80x9cFlexiboltxe2x80x9d(trademark) or at least provides a useful alternative.
Accordingly, one broad form of the invention which may be preferred provides a cable bolt comprising a plurality of generally closely spaced elongate wires or strands, said wires or strands having a first end adapted for fitting into a bore hole and a second end being terminated within a cylindrical sleeve, said cylindrical sleeve having a threaded external surface to receive a lock nut for tightening against a bearing plate so as to tension said cable bolt.
Preferably said termination of said second end of said wires or strands comprises a transverse plate in said cylindrical sleeve, said transverse plate having individual holes for each wire or strand and each wire or strand passes through a respective termination hole and has an enlarged head which is larger than the diameter of the termination hole whereby the wire or strand cannot be withdrawn from the said respective termination holes.
Preferably said wires or strands are separated outwardly from a central longitudinal axis of the cable bolt at discrete spaced locations along the length of the cable bolt to provide a xe2x80x9cbird cagexe2x80x9d at each location.
In one more specific form of the invention the wires or strands are straight and parallel to the longitudinal axis between each bird cage and in an alternative form said wires or strands have a slight helical configuration over the length of the cable bolt.
Preferably, coupling means such as a drive nut suitable for engagement with an industry standard square or hexagonal drive is provided in the lower end of said sleeve for the purpose of rotating said cable bolt.
In one more specific form said lock nut has a connection for receiving a grout tube and said connection provides access to an annular chamber formed within said lock nut, said annular chamber being against said bearing plate and said bearing plate having a bore therethrough which is greater in diameter than the outer diameter of said sleeve whereby said annular chamber is open to a space between said cable bolt and a wall of a bore hole in the mine roof into which the cable bolt is inserted, whereby grout is pumped through said grout tube and enters said bore hole in the mine roof.
In an alternative more specific form of the invention said grout enters said annular chamber via said coupling means.
Preferably, located along the longitudinal axis of said cable bolt is a central elongate tubular member which may be hollow and which extends over a substantial length of said cable bolt to act as an air bleed tube when grout is pumped into said bore hole, said tube extending through said sleeve and coupling means.
Preferably, a resin seal is provided on said cable bolt at a spaced location from said first end of said wires or strands to prevent resin, used to bond the first end of said wires or strands to the wall of the bore hole, from passing downwardly below said seal, said resin seal comprising a sleeve that fits over said wires or strands and said elongate tubular member terminating within said resin seal.
Preferably, spacers are provided at spaced locations around said elongate tubular member to cause said wires or strands to spread outwardly and form said bird cages.
Another preferred form of the invention provides a cable bolt for use in an excavation comprising a plurality of generally closely spaced elongate wires or strands having a first end adapted for fitting into a bore hole formed in said excavation and a second end adapted for bearing against the excavation surface wherein the terminal region of said second end of said wires or strands terminates in a cylindrical sleeve having a threaded external surface adapted to co-operate with a threaded lock nut for tightening said cable bolt against said excavation.